The present invention relates to a semiconductor device having a low-profile structure and mainly used in a memory card, an IC card, or the like.
In recent years, a semiconductor device having a low-profile structure is being strongly demanded to be mounted in a memory card, an IC card, or the like. FIG. 4 shows a conventional semiconductor device of this type. A UVEPROM (an ultraviolet erasable read only memory) semiconductor element 206 is fixed on a black alumina base 202 having a recessed portion 210 with a brazing material 207 such as a glass Ag paste or an Al paste, and the electrodes (not shown) of the semiconductor element 206 are connected to leads 203 through wiring lines 208. The resultant structure is covered with a translucent alumina cap 201, and is hermetically sealed with low-melting glass 204, thereby obtaining a package body having a thickness of 1.3 to 1.5 mm (Japanese Utility Model Laid-Open No. 1-160860). In addition, as another conventional example, there is a semiconductor device in which grooves 209 are formed in the sealing surface of a translucent alumina cap 201 as shown in FIG. 5, or the sealing surface of the translucent cap 201 is roughened. In this example, the contact area between the translucent alumina cap 201 and low-melting glass 204 is increased to improve the bonding strength between the cap and the low-melting glass, thereby improving the package strength (Japanese Utility Model Laid-Open No. 2-96739). As still another conventional example, there is a semiconductor device whose sealing portion is improved. This example is shown in FIG. 6. In this example, a layer consisting of a refractory glass layer 205 is formed at a boundary portion between a translucent alumina cap 201 and low-melting glass 204. The refractory glass 205 is screen-printed on the translucent alumina cap 201, and is sintered at a high temperature. When the glass is treated at a high temperature, since the glass sufficiently conforms to the recessed and projection portions of the surface of a ceramic material, a high bonding strength between the translucent alumina cap 201 and the refractory glass 205 can be obtained. Since the refractory glass 205 and the low-melting glass 204 consist of glass materials, the refractory glass 205 tends to be properly mixed with the low-melting glass 204, and a sufficient bonding strength can be obtained (Japanese Patent Laid-Open No. 4-6857).
As has been described above, a conventional semiconductor device whose package body has a thickness of 1.3 to 1.5 mm and a high bonding strength between the translucent alumina cap 201 and the low-melting glass 204 is known.
When the thickness of a package body is decreased to 1.3 mm or less by using the prior art, three problems, i.e., base cracking, base peeling, and cap cracking are posed.
Since the thin package body has a low mechanical strength not to withstand an external force, these problems are posed. Especially, the pressure (generally, 4 to 6 kgf/cm.sup.2) of a helium gas prior to a hermeticity test uniquely performed for a hermetic sealed package is the largest external force acting on the package and often breaking the package. This phenomenon occurs because of the following reasons. That is, when the package is pressed at a high pressure around the package during the pressurization of helium, as shown in FIG. 7, the central portions of the cap and base are recessed. This deformation causes the bottom portion of the base, the boundary portion between the base and the low-melting glass, the upper portion of the cap, and the boundary portion between the cap and the low-melting glass to generate high stresses, as indicated by arrows A, B, C, and D in FIG. 7, respectively. Base cracking, base peeling, cap cracking, and cap peeling occur at the bottom portion of the base, the boundary portion between the base and the low-melting glass, the upper portion of the cap, and the boundary portion between the cap and the low-melting glass, respectively.
On the other hand, the package strength of the above four portions of the conventional UVEPROM semiconductor device is very low except for the boundary portion between the cap and the low-melting glass upon the above counter measure.
First, the base portion has the following drawbacks:
Since the base portion has a recessed shape, the central portion having a small thickness has a low strength.
Since black alumina serving as the material of the base contains Al.sub.2 O.sub.3 having a low purity of 90 to 92% and contains a glass component such as SiO.sub.2 and MgO having low strength, the glass portion is easily broken first, thereby degrading the strength of the base.
Second, the boundary portion between the base and the low-melting glass has the following drawback:
When the thickness of the base is decreased, the deformation amount of the base is increased to increase a load on the boundary portion. However, since a countermeasure for improving the bonding strength between the base and the low-melting glass is not performed, the strength of the boundary portion between the base and the low-melting glass is degraded.
Third, the cap portion has the following drawback:
Translucent alumina serving as the material of the cap has a large grain size of 10 to 30 .mu.m and an average grain size of 15 .mu.m, and crystal grain intervals of the translucent alumina are wide. For this reason, the cap has many voids, and cracking easily occurs from the voids, thereby degrading the strength of the cap. As described above, the conventional semiconductor device has the drawbacks at the three portions.